Leaky Gut Syndrome: A Functional Nutritionist's Guide to Root Causes, Testing and Treatment

By Rita Soares, Functional Nutritionist

You have been told your gut is fine. Your gastroscopy came back normal. Your GP ran the standard bloods and found nothing remarkable. And yet you are still bloated after almost every meal, exhausted in ways that sleep does not fix, dealing with skin flares, brain fog, or joint pain that nobody has been able to connect to anything.

There is a mechanism that explains this pattern in a significant number of people — and it does not appear on a standard NHS or GP panel. It is called increased intestinal permeability, or what is more commonly known as leaky gut. It is measurable, it is clinically significant, and it is treatable when addressed properly.

This article explains what increased gut permeability actually is, what damages the gut lining, which symptoms it causes (including the ones that seem entirely unrelated to digestion), how it is tested in a functional nutrition context, and what a structured repair protocol looks like.

What Is Increased Gut Permeability or Leaky Gut?

The intestinal lining is not simply a passive tube. It is a highly selective, single-cell-thick barrier — roughly 30 square metres in a healthy adult — whose job is to allow nutrients, water, and electrolytes into the bloodstream while keeping bacteria, undigested food particles, toxins, and bacterial byproducts firmly out.

This selectivity is maintained by structures called tight junctions: protein complexes that seal the gaps between adjacent intestinal cells. Think of them as the mortar between bricks. When tight junctions are functioning correctly, only what belongs in circulation gets through.

When tight junctions are compromised — through chronic inflammation, dysbiosis, certain foods, medications, or sustained stress — the barrier becomes more permeable than it should be. Particles that the immune system would normally never encounter begin translocating into the portal circulation. The immune system, encountering these foreign antigens, mounts a response. That response does not stay local to the gut. It becomes systemic.

This is the mechanism by which a gut problem generates symptoms that appear to have nothing to do with digestion.

The research is substantial. A 2024 narrative review in Internal and Emergency Medicine documented the relationship between intestinal permeability, gut microbiota disruption, and systemic inflammation across multiple chronic disease categories. The gastroenterology literature has been examining this mechanism for over a decade, and our understanding of the specific proteins involved — particularly zonulin — has made it increasingly measurable.

What Damages the Gut Lining?

Understanding the root causes of increased permeability is essential because treating symptoms without addressing these drivers produces short-term improvement at best and no lasting change at worst. In clinical practice, the following are the most consistent contributors.

Dysbiosis and Microbial Imbalance

The gut microbiome plays a direct role in maintaining barrier integrity. Beneficial bacteria produce short-chain fatty acids — particularly butyrate — which serve as the primary fuel source for colonocytes, the cells lining the colon. When the microbiome is disrupted, butyrate production declines, the gut lining becomes structurally compromised, and pathogenic bacteria produce higher levels of lipopolysaccharide (LPS), an inflammatory compound found in the outer membrane of gram-negative bacteria. Elevated circulating LPS is one of the most reliable indicators that the gut barrier has been breached.

Dysbiosis itself is driven by antibiotic use, a low-fibre Western diet, chronic stress, and environmental exposures — meaning the most common features of modern life are actively working against barrier integrity.

Diet: Gluten, Refined Sugars, Emulsifiers and Alcohol

The wheat protein gliadin directly upregulates zonulin — the primary protein regulator of tight junction permeability — even in people without coeliac disease. This does not mean everyone needs to eliminate gluten permanently, but it is a clinically relevant mechanism that explains why some people feel substantially better when gluten is temporarily removed.

Refined sugars and ultra-processed foods increase intestinal inflammation and promote dysbiosis. Emulsifiers — widely used in processed foods to extend shelf life — have been shown to disrupt the mucus layer that protects the epithelium. Alcohol has direct tight junction toxicity and actively prevents barrier restoration while consumption continues.

Chronic Psychological and Physiological Stress

Stress activates the release of corticotropin-releasing factor (CRF), which increases intestinal permeability through a well-documented neuroimmune mechanism. This is one of the reasons that people with anxiety or high chronic stress frequently present with digestive symptoms — the gut-brain axis runs in both directions, and a dysregulated stress response has measurable effects on barrier function.

Physiological stress has similar consequences. Endurance athletes, for example, have a high incidence of leaky gut — high-intensity sustained exercise redirects blood flow away from the gut, increasing permeability. This is clinically relevant for high-performing clients who exercise heavily but struggle with chronic gut symptoms and fatigue.

Medications: NSAIDs, Antibiotics and Oral Contraceptives

Chronic use of non-steroidal anti-inflammatory drugs (NSAIDs) — ibuprofen, aspirin, naproxen — is one of the most well-established pharmaceutical contributors to increased gut permeability. They inhibit prostaglandin synthesis, which disrupts the protective mucus layer and increases paracellular permeability.

Frequent antibiotic use reduces microbial diversity and disrupts the butyrate-producing species that maintain barrier integrity. Long-term oral contraceptive use alters the microbiome in ways that can compromise gut barrier function in susceptible individuals.

Hyperglycaemia and Metabolic Dysregulation

Chronically elevated blood glucose — even subclinical hyperglycaemia that does not yet meet diagnostic thresholds — has been shown to disrupt intestinal epithelial organisation and increase permeability. This creates a bidirectional relationship: metabolic dysfunction damages the gut, and a damaged gut drives systemic inflammation that worsens insulin resistance.

Symptoms That Point to Increased Permeability

This is the part most people find most useful — and most surprising. Increased gut permeability does not always announce itself with digestive symptoms. In many cases, the gut itself feels relatively tolerable while the systemic consequences are the primary complaint.

Digestive symptoms that frequently accompany increased permeability include: bloating, particularly after meals regardless of what is eaten; irregular bowel habits; abdominal discomfort; food intolerances that seem to be multiplying; and acid reflux.

Systemic symptoms — those that appear disconnected from the gut — include:

• Chronic fatigue that is disproportionate to lifestyle and does not resolve with sleep

• Brain fog: difficulty concentrating, word retrieval issues, cognitive sluggishness

• Skin conditions: eczema, psoriasis, rosacea, and acne have all been associated with systemic immune activation triggered by translocated gut antigens

• Joint pain and inflammation without an identified rheumatological cause

• Autoimmune conditions: increased gut permeability is documented in coeliac disease, Hashimoto's thyroiditis, rheumatoid arthritis, type 1 diabetes, and multiple sclerosis — in some cases appearing to precede diagnosis

• Mood disturbance: anxiety and depression have been linked to neuroinflammation driven by LPS-mediated immune activation

• Hormonal dysregulation: the estrobolome — the subset of gut bacteria responsible for oestrogen metabolism — is directly affected by dysbiosis and barrier disruption, with measurable effects on circulating oestrogen levels

• Weight and metabolic issues: LPS-driven inflammation impairs leptin and insulin signalling, contributing to appetite dysregulation and metabolic dysfunction

  
  

What makes increased intestinal permeability particularly challenging to identify in conventional medicine is that all of these symptoms are typically investigated and managed as separate conditions. The gut is not looked at as the common driver. This is precisely where a functional nutritionist's approach differs — and where the most significant clinical breakthroughs tend to occur.

How Is Gut Permeability Tested?

Standard NHS or GP blood panels do not assess intestinal permeability. These are specialised markers that need to be specifically requested and are available through functional nutritionists and certain private laboratories.

Zonulin (Serum or Stool)

Zonulin is the primary protein that regulates tight junction permeability. Elevated zonulin confirms that the barrier is actively being disrupted. Serum zonulin is the most commonly assessed marker and has been documented as elevated in coeliac disease, IBS, type 1 diabetes, non-alcoholic fatty liver disease, metabolic syndrome, and multiple sclerosis.

Interpretation requires clinical context: zonulin values vary by laboratory platform, and results should be considered alongside symptoms and complementary markers rather than in isolation.

LPS and LPS-Binding Protein (LBP)

Lipopolysaccharide (LPS) is the inflammatory endotoxin produced by gram-negative bacteria. Its presence in the bloodstream is a direct indicator that bacteria or bacterial fragments are translocating through a compromised gut wall. LPS-binding protein is often easier to measure and serves as a reliable proxy.

Elevated LPS in systemic circulation drives a sustained low-grade inflammatory response and is associated with metabolic endotoxaemia — a condition now being studied in the context of obesity, type 2 diabetes, and cardiovascular disease.

Lactulose-Mannitol Ratio (Urine)

This is a functional permeability test. Two sugar molecules — lactulose (large) and mannitol (small) — are ingested orally. Mannitol is normally absorbed easily; lactulose should pass through with difficulty. An elevated lactulose-to-mannitol ratio in a subsequent urine collection indicates increased paracellular permeability — the gut is letting larger molecules through than it should.

Comprehensive Stool Analysis

A detailed stool test (such as the GI-MAP or equivalent) provides a microbiome overview alongside markers of gut inflammation, secretory IgA levels (the gut's first-line immune defence), butyrate-producing species, dysbiosis patterns, and stool zonulin. This gives a far more complete picture of what is driving the permeability and what a targeted intervention needs to address.

A Functional Nutritionist's Approach to Treatment

A functional nutrition protocol for increased gut permeability works in layers — and it is always built around the individual, not a generic template. What drives permeability in one person is not what drives it in another. What is needed to restore the barrier in one clinical picture may be entirely different from the next. This is why a thorough case history, symptom mapping, and targeted functional testing come before any intervention decisions.

That said, the framework follows a consistent logic.

Step 1: Identify and Remove the Drivers

This step is non-negotiable and is the most commonly missed in self-directed gut healing attempts. No intervention will restore a barrier that is being continuously damaged by an unaddressed driver — whether that is active dysbiosis, a medication contributing to permeability, sustained psychological stress, or a dietary pattern that is chronically triggering zonulin release.

The specific drivers vary by person. For some, the primary issue is unresolved SIBO producing ongoing inflammation at the mucosal level. For others, it is chronic NSAID use, alcohol, a dysbiotic microbiome shaped by repeated antibiotic courses, or a pattern of eating that is continuously triggering tight junction disruption. Identifying which drivers are operating — rather than assuming — is what makes the difference between a protocol that works and one that does not.

Step 2: Restore Microbial Balance

Because dysbiosis is both a consequence and a driver of increased permeability, addressing the microbiome is central to any serious gut healing protocol. A 2025 systematic review and meta-analysis across 68 clinical trials found that targeted microbiome support produced significant improvements in intestinal permeability markers. What this looks like in practice — which strains, which substrates, which timeline — depends entirely on what the stool analysis reveals about the individual's microbial landscape.

Step 3: Support Mucosal Repair

There is good evidence for a range of nutritional compounds that support tight junction integrity, mucosal barrier repair, and the reduction of LPS-driven inflammation. The specific interventions used in a clinical protocol are always selected based on the individual's test results, symptom picture, health history, and any relevant medication or nutrient interactions. This is not an area where a generic list applies — the right support for one person may be unnecessary or even inappropriate for another.

Step 4: Reintroduce and Monitor

As barrier function improves, a systematic reintroduction of any eliminated foods allows for accurate identification of true sensitivities versus permeability-driven reactions. Many people find that foods they had been reacting to — that appeared to be fixed intolerances — resolve as gut integrity is restored. Retesting objective markers 3–6 months into a protocol provides evidence of progress and guides decisions about continuing or adjusting the approach.

Why This Is Rarely Addressed in Conventional Medicine

Conventional medicine is exceptionally good at identifying acute, measurable pathology. A gastroscopy will find ulcers, polyps, and tumours. A colonoscopy will identify inflammatory bowel disease. Standard blood panels will detect coeliac antibodies, anaemia, and elevated inflammatory markers.

What these tools do not assess is barrier function — the degree to which the gut is selectively permeable — or the downstream systemic consequences of low-grade, chronic antigen translocation. Zonulin is not on a standard NHS request form. LPS-binding protein is not part of a routine private health screen. And the clinical picture that increased gut permeability produces — fatigue, brain fog, joint pain, skin issues, mood disturbance, multiple food sensitivities — tends to generate multiple specialist referrals rather than a unified investigation of the gut as the common driver.

This is not a criticism of conventional medicine. It is an observation about where its tools are focused, and where the gaps lie. Functional nutrition fills those gaps by asking a different question: not "which disease does this patient have?" but "what mechanism is producing all of these symptoms, and how do we address it at the root?"

Frequently Asked Questions

Is leaky gut a real medical condition?

Increased intestinal permeability is a well-documented physiological phenomenon with substantial peer-reviewed research behind it, including publications in Nature Reviews Gastroenterology and Hepatology, Gut, and Gastroenterology. It is not a discrete diagnosis in the ICD coding sense, which is partly why it remains outside routine clinical investigation. The mechanism is real, it is measurable, and the consequences are clinically significant.

Can leaky gut cause autoimmune disease?

The research here is compelling but not fully resolved. Increased gut permeability is consistently documented in a range of autoimmune conditions and, in some cases, appears to precede clinical diagnosis. The most widely cited research comes from Dr. Alessio Fasano, whose work on zonulin established that barrier dysfunction is a feature of coeliac disease and is documented across multiple autoimmune conditions. Whether permeability is a cause or a consequence in each case is condition-dependent, but restoring barrier integrity is a rational and clinically useful intervention regardless.

How long does gut healing take?

This is highly individual and depends on how long the barrier has been compromised, the severity of underlying dysbiosis and how consistently the protocol is followed. In clinical practice, most people begin to notice meaningful symptomatic improvement within 6–12 weeks of a structured protocol. Objective marker improvement — as measured by zonulin retesting — typically takes 3–6 months. Full restoration, particularly where there is significant autoimmune involvement, may take longer.

Do I need to test before starting a gut healing protocol?

Testing is not mandatory, but it significantly improves the precision and effectiveness of the intervention. Knowing your zonulin level, your microbiome composition, your sIgA status, and whether LPS is elevated allows for a targeted protocol rather than a generic approach. For people who have already tried gut-healing strategies without lasting success, testing is particularly valuable — it reveals what has been missed.

Can I address this without working with a practitioner?

Some aspects of a gut healing protocol — dietary changes, reducing unnecessary NSAID use, reducing alcohol, increasing fibre — can be self-directed. The limitation is that without testing, you cannot know which specific drivers are operating in your case, which probiotic strains are most relevant, or whether the protocol is working at a biological level. Working with a practitioner who can interpret the markers and adjust the intervention accordingly produces substantially better outcomes than a self-directed approach based on general guidance.

When to Seek Clinical Investigation

Consider a functional nutrition assessment for intestinal permeability if you are experiencing:

• Chronic bloating, irregular bowel habits, or multiple food sensitivities that have not responded to standard dietary advice

• Fatigue and brain fog that is disproportionate to lifestyle and not explained by standard blood tests

• A diagnosed autoimmune condition and a desire to understand the gut's role in its management

• Skin conditions — particularly eczema, psoriasis, or rosacea — that have not responded to topical treatment alone

• Mood disturbance — anxiety or depression — especially if accompanied by digestive symptoms

• A pattern of symptoms across multiple body systems with no unifying diagnosis from conventional investigation

  
  

Work With a Functional Nutritionist for Leaky Gut

Rita works with clients internationally through online and in-person consultations, combining detailed case history, functional testing and personalised protocols built around what your results and your symptoms actually show — not a generic gut healing template.

If this pattern resonates, the starting point is a consultation to review your full history, identify the most relevant tests, and build a protocol that addresses the specific drivers operating for you.

Book your consultation →

Scientific References

1. Di Vincenzo, F., Del Gaudio, A., Petito, V., Lopetuso, L.R., & Scaldaferri, F. (2024). Gut microbiota, intestinal permeability, and systemic inflammation: A narrative review. Internal and Emergency Medicine, 19, 275–293.

2. Horowitz, A., Chanez-Paredes, S.D., Haest, X., & Turner, J.R. (2023). Paracellular permeability and tight junction regulation in gut health and disease. Nature Reviews Gastroenterology & Hepatology, 20, 417–432.

3. Camilleri, M. (2019). Leaky gut: Mechanisms, measurement and clinical implications in humans. Gut, 68, 1516–1526.

4. Zmora, N., Suez, J., & Elinav, E. (2019). You are what you eat: Diet, health and the gut microbiota. Nature Reviews Gastroenterology & Hepatology, 16, 35–56.

5. Thaiss, C.A., et al. (2018). Hyperglycemia drives intestinal barrier dysfunction and risk for enteric infection. Science, 359(6382), 1376–1383.

6. Michielan, A., & D'Incà, R. (2015). Intestinal permeability in inflammatory bowel disease: Pathogenesis, clinical evaluation, and therapy of leaky gut. Mediators of Inflammation. doi:10.1155/2015/628157

7. Slyepchenko, A., et al. (2017). Intestinal microbiota, gut permeability and psychiatric disorders: implications for treatment. Current Pharmaceutical Design, 22(40), 6087–6106.

8. Fasano, A. (2012). Leaky gut and autoimmune diseases. Clinical Reviews in Allergy & Immunology, 42(1), 71–78.

9. Reinforcing gut integrity: A systematic review and meta-analysis of clinical trials assessing probiotics, synbiotics, and prebiotics on intestinal permeability markers. Pharmacological Research, 2025.

10. Moreira, A.P.B., Texeira, T.F.S., Ferreira, A.B., Peluzio, M.C.G., & Alfenas, R.C.G. (2021). Leaky gut: Effect of dietary fiber and fats on microbiome and intestinal barrier. International Journal of Molecular Sciences.